A new isozyme of human triosephosphate isomerase (TPI-A) is produced when lymphocytes undergo blast transformation. TPI-A is not normally found in adult human tissues but is produced in cells which have undergone transformation. It has been shown that TPI-A is a distinct gene product different from normal TPI-B, and it is possible that it may represent a fetal form of the enzyme. Several phosphoglucose isomerase (PGI) genetic variants have been studied in detail and methods developed for isolating the peptides at the subnanomole level have been developed. By using matrix-bound techniques, it was demonstrated that monomers of PGI are inactive but still bind substrate. Hybrids with one normal and one abnormal substrate are "half-active".